1. Field of the Invention
The invention relates to an optical fibre having at least two cores for propagating radiation. The invention also relates to a method of making such a fibre.
2. Discussion of Prior Art
Optical fibres with two cores have been reported and applied to various problems in optical sensing and telecommunications. The potential of multicore fibres with more than two single-mode cores has also been recognized [S. Prasad: "Focusing light into a multiple-core fiber II application to ground-based interferometry", Optics Communications 115 (1995) 368-378]. For example, bend sensing in the plane has been demonstrated using a four-core fibre, using a technique that can readily be extended to the monitoring of bending in three dimensions [M. J. Gander et al., "Bend measurements using multicore optical fiber", presented at OFS 12 1997 ].
However, at present there are relatively few applications for multicore optical fibres. This is because it is very difficult to make multicore fibres with the required uniformity and geometrical accuracy. One way which has been reported is to use an extension of the process previously used to make two-core fibres. In this method, single-core preforms made by standard vapour-phase techniques are precisely machined and are then assembled to form a multicore preform. The multicore preform is then drawn into fibre [G. Le Noane et al., "Ultra high density cables using a new concept of bunched multicore monomode fibers: A key for future FTTH networks", Proceedings of the International Wire & Cable Symposium (1994) 203-210]. This process, starting from a number of single core preforms, is a time-consuming and costly process. Furthermore, the addition of each extra core adds further complexity to the multicore preform. Higher multiplicity core fibres are therefore increasingly difficult to fabricate.
A single-mode photonic crystal fibre (PCF) is described by J. C. Knight et al., "All silica single-mode optical fiber with photonic crystal cladding", Optics Letters 21 (1996) 1547-1549 and is of relevance to the present invention.